core: Fix incorrect substitution of env vars in directives containing multiple env vars.

In ap_resolve_env(), the string returned from getenv() should be copied since
the returned string may be statically allocated.

This fixes an issue where the value for the last env var is substituted for all
env vars in a directive containing multiple env vars.

Submitted by: hwibell
Reviewed by: hwibell, covener, jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1848686 13f79535-47bb-0310-9956-ffa450edef68